1. Field of the Invention
The present invention relates to a porous hydroxyapatite which can be used as a biomaterial, and to a preparation method thereof.
2. Description of the Background Art
It has been known that an apatite exhibits an excellent bioactivity and bone conduction, and thus, has been widely used as a bio-ceramic. It is advantages since it is similar to bones in composition compared to other bio-ceramics such as a bioglass or A-W glass.
Bioactive ceramics that have been used as bone replacement materials, i.e., CaO, SiO2, MgO-based glass ceramics, contain significant amount of silicon and magnesium ions. Kokubo et al. has presented a theory that silicon existing in the glass ceramics is gradually released in a simulated body fluid to become a silicate ion present on a surface of the glass ceramics, and such silicate ion makes a new apatite nucleus be formed, and therefore, an apatite layer is formed quickly on the surface of the glass ceramics (Bioceramics vol. 7, 49, 1994). Carlisle et al. has discovered through an electron microprobe research that silicon plays a critical role for generation of bones. That is, chemical analysis results showed that newly formed bones always contain about 0.5% of silicon (Science, vol 167, 279, 1970), which supports the Kokubo's theory.
In order the ceramic materials to be used as a substitute material for bone, it should be quickly associated with live bones. For this purpose, a bone substitute material should have porous structure in which pores in size of 300–500 μm are three-dimensionally connected with each other. This is required because when the bone substitute material is actually implanted into a body, a body fluid can freely pass through the pores so that a new bone can be generated.
A natural coral is similar to a human cancellous bone in structure that it comprises calcium carbonate and has three-dimensionally connected pores in size of 200–500 μm. A method for converting a natural coral into an apatite by performing a hydrothermal treatment while maintaining its microstructures has been reported (U.S. Pat. Nos. 3,890,107 and 3,929,971; Biomaterials 17(17), p1709, 1996; and Material Characterization 47(2), p83, 2001).
Since a coral-derived natural apatite bone replacement material (Pro Osteon 200, 500, 200R, 500R) has become available from Interpore Co. in 1995 and used in a general surgical operation method, interests in and researches into the porous implant using a synthetic apatite have been accelerated.
There has been known a method for preparing a composite calcium phosphate of hydroxyapatite, tricalcium phosphate, tetracalcium phosphate, etc. from a natural coral showing an increased bioactivity and biodegradation rate, by changing reaction temperature, time, etc. of the hydrothermal process (U.S. Pat. No. 4,861,733 and European Patent Application No. 0,278,583). In addition, a method for converting only surface of the coral into hydroxyapatite (U.S. Pat. No. 4,976,736) has been proposed. In those methods, Ca/P ratios in calcium phosphate have been changed to vary the composition of the calcium phosphate, thereby to render a bioactivity to the hydroxyapatite.
However, in the apatite consisting of actual bones of a body, some of Ca, P and O sites are substituted with a small amount of other ions. In this case, even if a small amount of other ions are substituted, the substituted ions can make a great influence on surface charge, surface structure, strength, solubility, etc. of the apatite. In order to develop materials being similar to actual bones in composition, researches for preparing an apatite containing various ions have been performed.
It has been known that silicon exists as silicate ion, which has a tetrahedral structure, in a hydroxyapatite structure. There has been reported an example that P site of the hydroxyapatite has been substituted with silicon (European Patent Application No. 0,951,441; WO 98/08773; and U.S. Pat. No. 6,312,468). Besides, there has been also reported an example that Ca site of the hydroxyapatite has been substituted with magnesium (U.S. Pat. No. 6,585,946).
The present Inventors have reported a method for preparing hydroxyapatite powder containing silicon and magnesium ions using Ca(OH)2 and H3PO4 as starting materials and its sintering behavior in Biomaterials, vol 24, 1389, 2003.
In order to be used as a bone replacement material, a porous structure is required to be processed such that have pores in size of 300–500 μm which are three-dimensionally connected with each other.
In a conventional preparation method of a porous hydroxyapatite, hydroxyapatite slurry is infiltrated into a polyurethane foam and the resultant is then sintered. However, this method is not so much favorable, because volatile organic compounds generated during sintering process make a bad influence on the environment, as well as mechanical strength of the porous hydroxyapatite prepared by this method is very weak, which makes it difficult for the product to be stored, transported and used in surgical operation.
Meanwhile, since a natural coral is made of calcium carbonate and has three-dimensionally connected pores in size of 200–500 μm, it is similar to human cancellous bones in structure, so that it can be preferably used as a starting material for synthesis of hydroxyapatite. Another advantage is that mechanical strength of the hydroxyapatite prepared from a coral is very high.
It has been known that a comprehensive strength of the porous hydroxyapatite prepared from a natural coral is about 5.8 Mpa (Materials Science and Engineering C6, 175, 1998), which is much higher compared that that of the porous hydroxyapatite prepared by the conventional method as described above is about 1.3 Mpa.